fix: use confirm_modal macro instead of browser confirm for platform removal

Replaced the native browser confirm() dialog with the styled confirm_modal
macro for a consistent UI experience when removing platform assignments
from admin users.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/templates/admin/admin-user-edit.html

@@ -2,6 +2,7 @@
 {% extends "admin/base.html" %}
 {% from 'shared/macros/alerts.html' import loading_state %}
 {% from 'shared/macros/headers.html' import edit_page_header %}
+{% from 'shared/macros/modals.html' import confirm_modal %}
 
 {% block title %}Edit Admin User{% endblock %}
 
@@ -252,6 +253,18 @@
         </div>
     </div>
 </div>
+
+<!-- Remove Platform Confirmation Modal -->
+{{ confirm_modal(
+    'removePlatformModal',
+    'Remove Platform',
+    'Are you sure you want to remove this platform from this admin?',
+    'confirmRemovePlatform()',
+    'showRemovePlatformModal',
+    'Remove',
+    'Cancel',
+    'warning'
+) }}
 {% endblock %}
 
 {% block extra_scripts %}